Self-matching band-pass filter and related frequency down converter

ABSTRACT

A band-pass filter includes an input port, an output port, and a plurality of resonators. The input port is utilized for receiving a radio frequency signal. The output port is utilized for outputting a filtered signal. The plurality of resonators are placed between the input port and the output port, and are utilized for band-pass filtering the radio frequency signal for generating the filtered signal, wherein the plurality of resonators comprise at least two different trace widths for matching the output impedance of the band-pass filter with the input impedance of a rear-stage circuit coupled to the output port.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a band-pass filter and relatedfrequency down converter, and more particularly, to a self-matchingband-pass filter and related frequency down converter.

2. Description of the Prior Art

A satellite communication system capable of wideband and coverage iswidespread use in many areas, such as probe, military, telecommunicationnetwork, data communication, mobile communication, etc. A ground user ofthe satellite communication system requires a device consisting of anantenna, a low-noise block down-converter (LNB), and a demodulator forreceiving a satellite signal. After the satellite signal is received bythe antenna, the satellite signal is down converted to an intermediatefrequency (IF) signal via the LNB, and finally demodulated to atransmitted signal via the demodulator for outputting into a userdevice, such as a television.

Please refer to FIG. 1, which is a functional block diagram of an LNB 10according to the prior art. The LNB 10 includes a low noise amplifier100, a band-pass filter 102, a matching circuit 104, a mixer 106, and alocal oscillator 108. A radio frequency (RF) signal V_(RF) is receivedby an antenna, and enters the LNB 10. Then, the RF signal V_(RF) isamplified via the low noise amplifiers 100, and is filtered imagefrequency signal out via the band-pass filter 102 to generate a filteredsignal VF_(RF). Finally, the filtered signal VF_(RF) is down convertedto an IF band via the mixer 106 to output an IF signal V_(IF). Theband-pass filter 102 is a microstrip band-pass filter, which can beimplemented by the following filters: a hairpin band-pass filter, anend-coupled band-pass filter, and a parallel-coupled band-pass filter asshown in FIG. 2A-2C.

Please refer to FIG. 2A, which is a schematic diagram of a hairpinband-pass filter 20 according to the prior art. The hairpin band-passfilter 20 includes an input port PI_(A), an output port PO_(A) andresonators RN_(A) _(—) ₁-RN_(A) _(—) _(n). The input port PI_(A) and theoutput port PO_(A) are respectively coupled to a front-stage circuit anda rear-stage circuit for receiving and outputting signals. Trace width Wof each of the resonators RN_(A) _(—) ₁-RN_(a) _(—) _(n) is the same,and total length of each of the resonators RN_(A) _(—) ₁-RN_(a) _(—)_(n) is around half of a wavelength λ corresponding to a receivedsignal. FIG. 2B is a schematic diagram of an end-coupled band-passfilter 22 according to the prior art. The end-coupled band-pass filter22 includes an input port PI_(B), an output port PO_(B) and resonatorsRN_(B) _(—) ₁-RN_(B) _(—) _(n). The resonators RN_(B) _(—) ₁-RN_(B) _(—)_(n) are parallel lines and trace width W of each of the resonators RNB_(—) ₁-RN_(B) _(—) _(n) is the same. In addition, length L of each ofthe resonators RN_(B) _(—) ₁RN_(B) _(—) _(n) is λ/2. FIG. 2C is aschematic diagram of a parallel-coupled band-pass filter 24 according tothe prior art. The parallel-coupled band-pass filter 24 includes aninput port PI_(C), an output port PO_(C) and resonators RN_(C) _(—)₁-RN_(C) _(—) _(n). The resonators RN_(C) _(—) ₁-RN_(C) _(—) _(n) areparallel lines and trace width W of each of the resonators RN_(C) _(—)₁-RN_(C) _(—) _(n) is the same. In addition, length L of each of theresonators RN_(C) _(—) ₁-RN_(C) _(—) _(n) is λ/2, and adjacentresonators overlap for a length of λ/4. An amount of resonators of theabovementioned band-pass filters is related to filtering performance ofthe band-pass filters.

In order to cooperate a characteristic of a coaxial cable of ameasurement instrument, an input impedance to an output impedance of theband-pass filter 102 are usually set to 50Ω:50Ω. The mixer 106 isconstructed by active elements, such as a field effect transistor or abipolar junction transistor, whose input impedance is usually lower thanthe output impedance of the band-pass filter 102. The matching circuit104 is used for controlling impedance match between the band-pass filter102 and the mixer 106, so as to reduce the RF signal loss duringtransmission.

In the prior art, the rear-stage circuit elements are not particularlyconsidered for the input and output impedance designs of the band-passfilter. Therefore, in the conventional LNB, the output port of theband-pass filter needs to be coupled to the matching circuit, and thenperforms impedance match with the rear-stage circuit. However, when thematching circuit exists, transmission line effect cannot be decreased tothe lowest level and the loss of the RF signal outputted from theband-pass filter to the mixer cannot be improved effectively.

SUMMARY OF THE INVENTION

Therefore, the present invention provides a band-pass filter and relatedfrequency down converter, wherein the output impedance of the band-passfilter is matched with the input impedance of a rear-stage circuit.

The present invention discloses a band-pass filter which includes aninput port for receiving a radio frequency signal, an output port foroutputting a filtered signal, and a plurality of resonators placedbetween the input port and the output port, for performing band passfiltering on the radio-frequency signal to generate the filtered signal.The plurality of resonators comprise at least two different trace widthsfor matching the output impedance of the band-pass filter with the inputimpedance of a rear-stage circuit coupled to the output port.

The present invention further discloses a band-pass filter whichincludes an input port for receiving a radio frequency signal, an outputport for outputting a filtered signal, and a resonator placed betweenthe input port and the output port, for performing band pass filteringon the radio-frequency signal to generate the filtered signal. Tracewidth of the resonator is different from trace width of the input portfor matching the output impedance of the band-pass filter with the inputimpedance of a rear-stage circuit coupled to the output port.

The present invention discloses a down converter for a wirelesscommunication receiver which includes a mixer and a band-pass filter.The mixer is utilized for downconverting the frequency of a filteredsignal according to a local oscillating signal, for outputting anintermediate frequency signal. The band-pass filter is coupled to themixer and includes an input port for receiving a radio frequency signal,an output port for outputting a filtered signal, and a plurality ofresonators placed between the input port and the output port, forperforming band-pass filtering on the radio frequency signal to generatethe filtered signal. The plurality of resonators comprise at least twodifferent trace widths for matching the output impedance of theband-pass filter with the input impedance of a rear-stage circuitcoupled to the output port.

The present invention further discloses a down converter for a wirelesscommunication receiver which includes a mixer and a band-pass filter.The mixer is utilized for downconverting the frequency of a filteredsignal according to a local oscillating signal, for outputting anintermediate frequency signal. The band-pass filter is coupled to themixer and includes an input port for receiving a radio frequency signal,an output port for outputting a filtered signal, and a resonator placedbetween the input port and the output port, for performing band passfiltering on the radio-frequency signal to generate the filtered signal,wherein trace width of the resonator is different from trace width ofthe input port for matching the output impedance of the band-pass filterwith the input impedance of a rear-stage circuit coupled to the outputport.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of a satellite frequency downconverter according to the prior art.

FIG. 2A is a schematic diagram of a hairpin band-pass filter accordingto the prior art.

FIG. 2B is a schematic diagram of an end-coupled band-pass filteraccording to the prior art.

FIG. 2C is a schematic diagram of a parallel-coupled band-pass filteraccording to the prior art.

FIG. 3-FIG. 6 are schematic diagrams of band-pass filters according toembodiments of the present invention.

FIG. 7 is a functional block diagram of a low-noise block down converteraccording to an embodiment of the present invention.

FIG. 8 is a noise characteristic graph of a mixer shown in FIG. 7.

FIG. 9 is a conversion gain graph of a mixer shown in FIG. 7.

DETAILED DESCRIPTION

Please refer to FIG. 3, which is a schematic diagram of a band-passfilter 30 according to an embodiment of the present invention. Theband-pass filter 30 is a hairpin microstrip band-pass filter, whichincludes an input port PI₃, an output port PO₃, and resonators RN₃ _(—)₁-RN₃ _(—) ₃. The input port PI₃ is used for receiving an RF signal andthe output port PO₃ is used for outputting a filtered signal. Theresonators RN₃ _(—) ₁-RN₃ _(—) ₃ are U-shaped resonators and are placedbetween the input port PI₃ and the output port PO₃, which are arrangedas RN₃ _(—) ₁, RN₃ _(—) ₂, and RN₃ _(—) ₃ from the input port PI₃ insequence, for filtering the RF signal and generating the filteredsignal. Trace widths of the resonators RN₃ _(—) ₁-RN₃ _(—) ₃ aredesigned to match an output impedance of the band-pass filter 30 with aninput impedance of a rear-stage circuit of the band-pass filter 30 in acondition of knowing the input impedance of the rear-stage circuit withthe lowest noise.

The design of the band-pass filter 30 is illustrated as following. InFIG. 3, the input impedance of the rear-stage circuit is supposed lowerthan the input impedance of the band-pass filter 30. The closer to theoutput port PO₃, the larger trace widths of the resonators are, andthereby the trace widths are not the same, for matching the outputimpedance of the band-pass filter 30 with a lower input impedance of therear-stage circuit. Each of the resonators RN₃ _(—) ₁-RN₃ _(—) ₃includes two sections one closer to the input port PI₃ and anothercloser to the output port PO₃. The trace width of the section close tothe output port PO₃ are larger than the trace width of the section closeto the input port PI₃. A section P₃ _(—) ₁₁ of the resonator RN₃ _(—) ₁is closer to the input port PI₃, and another section P₃ _(—) ₁₂ iscloser to the output port PO₃, where the trace width W₂ of the sectionP₃ _(—) ₁₂ is larger than the trace width W₁ of the section P₃ _(—) ₁₁.Similarly, the resonator RN₃ _(—) ₂ includes two sections P₃ _(—) ₂₁ andP₃ _(—) ₂₂, and the trace width W₃ of the section P₃ _(—) ₂₂ is largerthan the trace width W₂ of the section P₃ _(—) ₂₁. The resonator RN₃_(—) ₃ includes two sections P₃ _(—) ₃₁ and P₃ _(—) ₃₂, and the tracewidth W₄ of the section P₃ _(—) ₃₂ is larger than the trace width W₃ ofthe section P₃ _(—) ₃₁. In other word, the resonators RN₃ _(—) ₁-RN₃_(—) ₃ have four different trace widths. Through gradually increasedtrace widths of the resonators RN₃ _(—) ₁-RN₃ _(—) ₃, the outputimpedance of the band-pass filter 30 can be matched with the lower inputimpedance of the rear-stage circuit.

Please refer to FIG. 4, which is a schematic diagram of a band-passfilter 40 according to an embodiment of the present invention. Theband-pass filter 40 is also the hairpin microstrip band-pass filter,which includes an input port PI₄, an output port PO₄, and resonators RN₄_(—) ₁-RN₄ _(—) ₃. The input port PI₄ is used for receiving an RF signaland the output port PO₄ is used for outputting a filtered signal. Theresonators RN₄ _(—) ₁-RN₄ _(—) ₃ is placed between the input port PI₄and the output port PO₄, which are arranged as RN₄ _(—) ₁, RN₄ _(—) ₂,and RN₄ _(—) ₃ from the input port PI₄ in sequence, for filtering the RFsignal and generating the filtered signal. Similar to the band-passfilter 30 of FIG. 3, in the band-pass filter 40, the closer to theoutput port PO₄, the larger trace widths of the resonators are, whichmeans the trace width W₃ of the resonator RN₄ _(—) ₃ is larger than thetrace width W₂ of the resonator RN₄ _(—) ₂, and the trace width W₂ ofthe resonator RN₄ _(—) ₂ is larger than the trace width W₁ of theresonator RN₄ _(—) ₁. The difference is that each of the resonators RN₄_(—) ₁-RN₄ _(—) ₃ has one trace width. In other words, in the band-passfilter 40, each resonator is not divided into sections with differenttrace widths, but with the same trace width.

As can be seen from above, the band-pass filter 30 and 40 both graduallyincrease the trace widths of the resonators for matching the outputimpedance with the lower input impedance of the rear-stage circuit.Similarly, the band-pass filters of the embodiments of the presentinvention can gradually decrease the trace widths of the resonators formatching the output impedance with a higher input impedance of therear-stage circuit. The input impedance and output impedance of theband-pass filter in the prior art are symmetrical, so the trace width ofeach resonator is the same. In comparison, the present invention canmake the output impedance of the band-pass filter dissymmetrical to theinput impedance through gradually changed trace widths of theresonators, and matching with the input impedance of the rear-stagecircuit, so as to economize on the matching circuit between theband-pass filter and the rear-stage circuit, and decrease the cost ofthe elements. Since the band-pass filter of the present invention hasbeen matched with the rear-stage circuit, which can be calledself-matching band-pass filter.

The difference between the band-pass filter 30 and 40 is that theband-pass filter 30 increases the trace width by a section of theresonator, and the band-pass filter 40 increases the trace width by aresonator. Please note that, in the embodiment of the present invention,the way of increasing the trace width by the resonator or the section ofthe resonator can be utilized in a band-pass filter at the same time,and can be utilized in some of the resonators according to therequirement. For example, a hairpin band-pass filter of the embodimentof the present invention includes three resonators divided into sixsections W₁-W₆which indicates trace widths of each section from theinput port to the output port, and a relationship between the tracewidths of each section is W₁<W₂<W₃<W₄<W₅<W₆, which can match the outputimpedance of the band-pass filter with the lower input impedance of therear-stage circuit also.

Note that, shapes of the input port or output port in FIGS. 3 and 4 arewide metal line connected to an upper point of a thin metal line whichcan be called a coupled line. In the embodiment of the presentinvention, the input port or output port of the hairpin band-pass filterconnects to a position of the coupled line, such as a middle point or alower point of the coupled line. In addition, in the hairpin band-passfilter of FIGS. 3 and 4, openings of U-shaped resonators are arranged ina way of reverse to each other. The way of opening arrangement is onlyan exemplary embodiment, and is not limited herein. The openings of twoadjacent U-shaped resonators of the hairpin band-pass filter can bearranged in the same direction or reverse directions. In other words,the openings of all U-shaped resonators may be arranged in the samedirection or reverse directions, or the openings of part adjacentU-shaped resonators are arranged in reverse directions. Shapes of theabovementioned input port and output port, and the opening directions ofthe U-shaped resonators can be freely designed, but the output impedanceof the band-pass filter has to be matched with the input impedance ofthe rear-stage circuit.

The gradual change trace width of the resonators is not only utilized inthe hairpin band-pass filter, but also in two types of microstripband-pass filter: parallel coupled band-pass filter and end-coupledband-pass filter. Please refer to FIG. 5, which is a schematic diagramof a band-pass filter 50 according to an embodiment of the presentinvention. The band-pass filter 50 is a parallel-coupled band-passfilter, which includes an input port PI₅, an output port PO₅, andresonators RN₅ _(—) ₁-RN₅ _(—) ₃. The operations of the input port PI₅,the output port PO₅, and the resonators RN₅ _(—) ₁-RN₅ _(—) ₃ aresimilar to elements of the abovementioned embodiment, so the detaileddescription is omitted herein. The resonators RN₅ _(—) ₁-RN₅ _(—) ₃ areplaced between the input port PI₅ and the output port PO₅, which arearranged as RN₅ _(—) ₁, RN₅ _(—) ₂, and RN₅ _(—) ₃ from the input portPI₅ in sequence. The resonators RN₅ _(—) ₁-RN₅ _(—) ₃ are parallel toeach other, and traces of adjacent resonators overlap. The closer to theoutput port PO₅, the larger trace widths of the resonators are. Each ofthe resonators RN₅ _(—) ₁-RN₅ _(—) ₃ includes two sections one closer tothe input port PI₅ and another closer to the output port PO₅. The tracewidth of the section closer to the output port PO₅ is larger than thetrace width of the section closer to the input port PI₅. Each section ofthe resonators RN₅ _(—) ₁-RN₅ _(—) ₃ in FIG. 5 is not numericallyassigned, and is marked with the width for clearly showing the gradualchange width. The widths of each section of the resonators RN₅ _(—)₁-RN₅ _(—) ₃ are W₁, W₂, W₂, W₃, W₃, and W₄ in sequence and a way ofwidth change is similar to the way of the band-pass filter 30 of FIG. 3.

Please refer to FIG. 6, which is a schematic diagram of a band-passfilter 60 according to an embodiment of the present invention. Theband-pass filter 60 is an end-coupled band-pass filter, which includesan input port PI₆, an output port PO₆, and resonators RN₆ _(—) ₁-RN₆_(—) ₃. The operations of the input port PI6, the output port PO₆, andthe resonators RN₆ _(—) ₁-RN₆ _(—) ₃ are similar to elements of theabovementioned embodiment, so the detailed description is omittedherein. The resonators RN₆ _(—) ₁-RN₆ _(—) ₃ are placed between theinput port PI₆ and the output port PO₆, which are arranged as RN₆ _(—)₁, RN₆ _(—) ₂, and RN₆ _(—) ₃ from the input port PI₆ in sequence andparallel to each other. Trace length L of each of the resonators RN₆_(—) ₁-RN₆ _(—) ₃ is the same, and the trace widths of each of theresonators RN₆ _(—) ₁-RN₆ _(—) ₃ are different. The relationship betweenthe trace widths is W₁<W₂<W₃, and the closer to the output port PO₆, thelarger trace widths of the resonators are.

The abovementioned band-pass filter 30, 40, 50, or 60 includes threeresonators as an example. In practice, regardless of the hairpin,parallel-coupled or end-coupled band-pass filter, the present inventiononly needs at least a resonator for realizing a goal of adjusting theoutput impedance. For the end-coupled band-pass filter including asingle resonator, the trace width of the resonator must be differentfrom the trace width of the input port for adjusting the outputimpedance. For hairpin band-pass filter and parallel-coupled band-passfilter including a resonator, which realizes adjustment of the outputimpedance through two sections with different trace widths in a singleresonator. On the other hand, the trace width of the single resonator isnot divided into sections, and is different from the trace width of theinput port for adjusting the output impedance. Take the hairpinband-pass filter 40 as an example, if the resonator RN₄ _(—) ₁ is theonly resonator, the trace width of resonator RN₄ _(—) ₁ is differentfrom the thin coupled line of the input port PI₄.

The present invention further applies the abovementioned band-passfilters to a conventional low-noise block down converter (LNB) of awireless communication receiver for reducing the matching circuitbetween the band-pass filter of the LNB and the rear-stage circuit.Please refer to FIG. 7, which is a functional block diagram of an LNB 70according to an embodiment of the present invention. The LNB 70 includesa low noise amplifier 700, a band-pass filter 702, a mixer 704, and alocal oscillator 706. The low noise amplifier 700 is used for amplifyingan RF signal V_(RF) received by an antenna. The band-pass filter 702 iscoupled to the low noise amplifier 700, and is used for filtering animage frequency signal out of the RF signal V_(RF) and then generating afiltered signal VF_(RF). The band-pass filter 702 is one of theabovementioned band-pass filters 30, 40, 50, 60, or any other band-passfilter similar to embodiments of the present invention. For the mixerutilized in the LNB, the input impedance of the mixer is around 5Ω-20Ωin the lowest noise situation, which is far smaller than the inputimpedance 50Ω of the band-pass filter 702. The gradual increase tracewidth of the resonator in the band-pass filter 702 is designed accordingto the input impedance of the rear-stage mixer 704. The mixer 704 doesnot need to couple to the matching circuit and is directly coupled tothe band-pass filter 702 and the local oscillator 706 for downconvertingfrequency of the filtered signal VF_(RF) to intermediate frequencyaccording a local oscillating signal generated by the local oscillator706, and outputting an IF signal V_(IF).

The LNB 70 is only an embodiment of the present invention, and theabovementioned band-pass filter can be utilized in other frequency downconverter of the wireless communication receiver. Please note that, thefiltered signal VF_(RF) generated by the band-pass filter 702 is notaffected by a transmission line effect of the external matching circuitto cause signal loss or noise interference. Therefore, noisecharacteristic and conversion gain of the mixer 704 is better than themixer of the frequency down converter in the prior art. Please refer toFIG. 8, which is a noise characteristic graph of the mixer 704 of theLNB 70 shown in FIG. 7, where a curve SI is a noise characteristic curveof the mixer of the frequency down converter in the prior art, and acurve S2 is a noise characteristic curve of the mixer 704. An average ofthe curve S2 is 1-2 dB lower than the curve S1, and noise flatness ofthe curve S2 is also better than the curve S1. Please refer to FIG. 9,which is a conversion gain graph of the mixer 704 of the LNB 70 shown inFIG. 7, where a curve C1 is a conversion gain curve of the mixer of thefrequency down converter in the prior art, and a curve C2 is aconversion gain curve of the mixer 704. As can be seen in FIG. 9, theconversion gain of the mixer 704 is increased substantially as a resultof the characteristic of the band-pass filter 702 that self-matches theoutput impedance.

In conclusion, the present invention matches the output impedance of theband-pass filter with the input impedance of the rear-stage circuit bygradually changed trace widths of the resonators. Moreover, theband-pass filter of the present invention can be utilized in thefrequency down converter for reducing the matching circuit, decreasingthe cost, and enhancing the output characteristic of the rear-stagemixer.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention.

1. A band-pass filter comprising: an input port for receiving a radiofrequency signal; an output port for outputting a filtered signal; and aplurality of resonators placed between the input port and the outputport, for performing band-pass filtering on the radio frequency signalto generate the filtered signal, wherein the plurality of resonatorscomprise at least two different trace widths for matching the outputimpedance of the band-pass filter with the input impedance of arear-stage circuit coupled to the output port; wherein each of theplurality of resonators comprises a first section and a second section,the trace width of the first section is different from the trace widthof the second section, and the trace width of the first section of theresonator closest to the output port is different from the trace widthof the second section of the resonator closest to the input port.
 2. Theband-pass filter of claim 1, wherein the trace width of the firstsection is larger than the trace width of the second section.
 3. Theband-pass filter of claim 1, wherein the trace widths of two adjacentresonators of the plurality of resonators are different.
 4. Theband-pass filter of claim 1, wherein the trace width of one of theplurality of resonators closed to the output port is larger than thetrace width of an adjacent resonator of the plurality of resonatorsclosed to the input port.
 5. The band-pass filter of claim 1, whereinthe band-pass filter is a hairpin band-pass filter.
 6. The band-passfilter of claim 5, wherein the band-pass filter comprises a plurality ofU-shaped resonators, opening direction of one of the plurality ofU-shaped resonators is different from opening direction of an adjacentU-shaped resonator of the plurality of U-shaped resonators.
 7. Theband-pass filter of claim 5, wherein the band-pass filter comprises aplurality of U-shaped resonators, opening direction of one of theplurality of U-shaped resonators is the same with opening direction ofan adjacent U-shaped resonator of the plurality of U-shaped resonators.8. The band-pass filter of claim 1, wherein the band-pass filter is aparallel-coupled band-pass filter.
 9. The band-pass filter of claim 1,wherein the band-pass filter is an end-coupled band-pass filter.
 10. Aband-pass filter comprising: an input port for receiving a radiofrequency signal; an output port for outputting a filtered signal; and aresonator directly placed between the input port and the output port,for performing band-pass filtering on the radio-frequency signal togenerate the filtered signal, wherein a trace width of the resonator isdifferent from a trace width of the input port for matching the outputimpedance of the band-pass filter with the input impedance of arear-stage circuit coupled to the output port.
 11. The band-pass filterof claim 10, wherein the resonator comprises a first section closed tothe input port and a second section closed to the output port, the tracewidth of the first section is different from the trace width of theinput port and the trace width of the second section.
 12. The band-passfilter of claim 11, wherein the band-pass filter is a hairpin band-passfilter or a parallel-coupled band-pass filter.
 13. The band-pass filterof claim 10, wherein the band-pass filter is an end-coupled band-passfilter.
 14. A down converter for a wireless communication receivercomprising: a mixer for downconverting the frequency of a filteredsignal according to a local oscillating signal, for outputting anintermediate frequency signal; and a band-pass filter, coupled to themixer, comprising: an input port for receiving a radio frequency signal;an output port for outputting a filtered signal; and a plurality ofresonators placed between the input port and the output port, forperforming band-pass filtering on the radio frequency signal to generatethe filtered signal, wherein the plurality of resonators comprise atleast two different trace widths for matching the output impedance ofthe band-pass filter with the input impedance of a rear-stage circuitcoupled to the output port; wherein each of the plurality of resonatorscomprises a first section and a second section, the trace width of thefirst section is different from the trace width of the second section,and the trace width of the first section of the resonator closest to theoutput port is different from the trace width of the second section ofthe resonator closest to the input port.
 15. The down converter of claim14, wherein the trace width of the first section is larger than thetrace width of the second section.
 16. The down converter of claim 14,wherein the trace widths of two adjacent resonators of the plurality ofresonators are different.
 17. The down converter of claim 14, whereinthe trace width of one of the plurality of resonators closed to theoutput port is larger than the trace width of an adjacent resonator ofthe plurality of resonators closed to the input port.
 18. The downconverter of claim 14, wherein the band-pass filter is a hairpinband-pass filter.
 19. The down converter of claim 18, wherein theband-pass filter comprises a plurality of U-shaped resonators, openingdirection of one of the plurality of U-shaped resonators is differentfrom opening direction of an adjacent U-shaped resonator of theplurality of U-shaped resonators.
 20. The down converter of claim 18,wherein the band-pass filter comprises a plurality of U-shapedresonators, opening direction of one of the plurality of U-shapedresonators is the same with opening direction of an adjacent U-shapedresonator of the plurality of U-shaped resonators.
 21. The downconverter of claim 14, wherein the band-pass filter is aparallel-coupled band-pass filter.
 22. The down converter of claim 14,wherein the band-pass filter is an end-coupled band-pass filter.
 23. Adown converter for a wireless communication receiver comprising: a mixerfor downconverting the frequency of a filtered signal according to alocal oscillating signal, for outputting an intermediate frequencysignal; and a band-pass filter, coupled to the mixer, comprising: aninput port for receiving a radio frequency signal; an output port foroutputting a filtered signal; and a resonator directly placed betweenthe input port and the output port, for performing band-pass filteringon the radio frequency signal to generate the filtered signal, wherein atrace width of the resonator is different from a trace width of theinput port for matching the output impedance of the band-pass filterwith the input impedance of a rear-stage circuit coupled to the outputport.
 24. The down converter of claim 23, wherein the resonatorcomprises a first section closed to the input port and a second sectionclosed to the output port, the trace width of the first section isdifferent from the trace width of the input port and the trace width ofthe second section.
 25. The down converter of claim 24, wherein theband-pass filter is a hairpin band-pass filter or a parallel-coupledband-pass filter.
 26. The down converter of claim 23, wherein theband-pass filter is an end-coupled band-pass filter.